The formation of synapses in early development is integral to structural development of postsynaptic targets. However, the mechanisms for these effects and the role of synaptic connections in functional maturation of the postsynaptic cell are not well understood, in part because both synaptic partners are not easily isolated for study. We propose experiments to study the role of synapse formation at the calyx of Held on functional maturation of its target, the principal neuron of the medial nucleus of the trapezoid body (MNTB). This system has unique advantages because both synaptic partners are large and experimentally accessible, and calyx growth and synaptic competition occurs in a compressed 2 day time period. We will take advantage of these characteristics to provide the most detailed description for the dynamics of competition among inputs in the CNS and coordinated maturation of pre and postsynaptic partners. In Aim 1, we determine the timing of presynaptic calyx growth relative to postsynaptic gene expression for ionic channels and neurotransmitter receptors and their appearance in the cell membrane as functional proteins. In Aim 2 we define the dynamics for competition and structural and functional characteristics of winning inputs. In Aim 3 we reveal the effects of surgical removal of neural inputs on functional maturation of MNTB cells and mechanisms by which Ca2+ signals in MNTB neurons and surrounding glia are activated by synaptic transmission. Our long term objectives are to understand how intercellular communication sculpts brain topography and induces neuronal maturation during nervous system development. The rationale for the proposed research is that knowledge of cellular events mediating synapse formation can translate into strategies for repair of nervous systems damaged by genetic mutation, injury and disease. We will employ a multidisciplinary set of techniques, including in vitro voltage and current clamp recordings, Ca2+ imaging, quantitative real-time PCR, confocal microscopy, large-scale serial section electron microscopy, electron tomography and 3-D reconstruction to accomplish the proposed aims.